Excel - hidden tab - protected file - cannot unhide tab

Hi all,

I had to protect a file with a hidden tab. Later I unprotected the file back and wanted to unhid the tab and work on it, but the Option to unhid is not anymore available (light Grey). 

What could be the Problem? The tab is still there, I can see the link in the formulas of the shown tab...

Thanks for your Support.

Angelika